1. Field of the Invention
The present invention relates to an optical scanner that scans a light flux from a light source to a photosensitive surface of each of a plurality of photoconductors in a main-scanning direction. In particular, the present invention relates to a technique of achieving reduction in space for allocation of an optical system and improvement of an optical characteristic of scan light.
2. Description of the Related Art
Conventionally, in relation to an optical scanning technique that scans a light flux from a light source to a photosensitive surface of each of a plurality of photoconductors in a main-scanning direction, there has been disclosed an image forming device that forms an image such as an electrostatic latent image on the photoconductors by irradiation of a light beam. The image forming device sets a plurality of reflecting surfaces in a rotational deflector that scans the light beam to the plurality of photoconductors to have an inclination angle different from one another with respect to a rotational axis, and scans the different photoconductor for each of the reflecting surfaces having the inclination angle different from one another (for example, refer to Patent Document 1: Jpn. Pat. Appln. Publication No. 2000-2846 and Patent Document 2: Jpn. Pat. Appln. Publication No. 11-218991).
In the conventional art configured as described above, a polygon mirror having the reflecting surfaces each having a reflecting angle different from one another is used, so that both kinds of scanning, that is, scanning by and switchover of a light path of a laser beam, can be carried out by a rotational operation of a polygon mirror. Therefore, lowering of cost by reduction in the number of items of parts and by reduction in movable parts, and printing with high precision by simplification of control operation can be achieved.
In the conventional art described above, there also is an invention in which a cylinder lens is arranged in a pre-deflection optical system (refer to Patent Document 1). In this invention, light is considered to be converged in the vicinity of a reflecting surface of the deflector in a sub-scanning direction, and in the post-deflection optical system, the reflecting surface and the image surface of the deflector are considered to be made to have a substantially conjugate relationship in the sub-scanning direction so as to provide a surface tilt correcting function. On the other hand, there is a configuration where the cylinder lens is not included in the pre-deflection optical system. In such a configuration, the post-deflection optical system is considered not to have the surface tilt correcting function (for example, refer to Patent Document 2 and Patent Document 3: Jpn. Pat. Appln. Publication No. 63-273814).
Conventionally, in a case where each of the reflecting surfaces of the polygon mirror is made to have an angle different from one another and a light beam is imaged onto each exposure target part by the post-deflection optical system that has the surface tilt correction function, each of the light beams deflected on each of the surfaces of the polygon mirror has been imaged in an optical part different from one another. In this case, as much as there is a need to secure certain space between the optical parts, there is a need to make the inclination angle applied to each of the reflecting surfaces of the polygon mirror large, and the space for allocating the optical system in the sub-scanning direction is enlarged.
In addition, by securing the certain space between the optical parts, generation of an asymmetric wave aberration which becomes larger when the inclination angle applied to each of the reflecting surfaces of the polygon mirror is large becomes difficult to restrict. This has led to deterioration in an imaging characteristic.
Further, in a case where the post-deflection optical system does not have the surface tilt correcting function, an allowable amount of surface tilt of each of the reflecting surfaces of the polygon mirror becomes extremely small, and there is a problem that manufacturing cost of the polygon mirror significantly increases. In addition, in a case where the lens does not have the surface tilt correcting function and the configuration is such that light beam passes through the common lens from each of the reflecting surfaces of the polygon mirror to which an angle is applied, a scan line is curved and overlapping of colors with high precision (registration) cannot be carried out. In particular, in a case where a full color image is formed by using four sets of beams in order to form four latent images, an image has been significantly deteriorated due to color shift.